122 INSTRUMENTAL METHODS
The reference 28 authors concluded that for the NMR solution study of
NNGH inhibited MMP12 (MMP12 - NNGH), the disorder exhibited arose
from the lack of medium - to long - range NOEs due to the protein ’ s backbone
(BB) mobility. The side - chain NOEs were strongly reduced in intensity, indi-
cating that these were very mobile as well. Residual dipolar couplings (RDCs),
potentially sensitive indicators of mobility in NMR structures, were found, for
the PDB: 1YCM NMR structure, to be reduced in their absolute values com-
pared to calculated values. The changes occurred particularly at hinges between
loops andα - helices or β - sheets, and in loop regions. In Figure 3.18A , these
regions can be identifi ed as: (1) between β 1 and L2, and L2 and α 1 (see
orange – red residues 122 – 124); (2) before β 2 (residues 146 – 147); (3) after β 2
(residue 155); (4) in L5 (residue 171); (5) in L7 (residue 207); and (6) in L8
(residues 244 – 248). Importantly, comparison of the loop regions in solid state
(X - ray) versus solution (NMR) indicated a high degree of similarity in regions
exhibiting mobility (see Figures 3.18A and 3.18B ). The reference 28 authors
also concluded, based on comparisons to other MMPs that have had their
NMR structures determined, that the overall profi les of the structures were
similar and that similar regions of high mobility were found in loop regions.
Overall, the authors concluded that comparisons of X - ray crystallographic and
NMR structures, as well as NMR structures of similar molecules, yield impor-
tant information about regions of instability in proteins. This information will
be useful in many regards; in the case of this study of MMP inhibitors,
the conclusions are important for researchers attempting to discover drug
molecules that could be clinically useful.
3.5 Electron Paramagnetic Resonance,
3.5.1 Theory and Determination of g-Values,
In electron paramagnetic resonance (EPR) spectroscopy, also called electron
spin resonance (ESR), radiation of microwave frequency is absorbed by
molecules, ions, or atoms (organic or inorganic) containing a paramagnetic
center — that is, a system with one or more unpaired electrons. In EPR, the
unpaired electron spin moment (m s = ± 1/2 for a free electron) interacts with
an applied magnetic fi eld producing the so - called Zeeman effect. The effect is
illustrated in Figure 3.19 for the electron spin functions α and β corresponding
toms = +1/2 and ms = − 1/2.
Three basic equations (3.34 – 3.36) are needed to describe the technique. In
the equations, μ is the magnetic moment of the electron, sometimes also
written as μ e , g is called the g factor or spectroscopic splitting factor, S is
defi ned as the total spin associated with the electron (in bold type because it
is considered as a vector), B is the imposed external magnetic fi eld (also
defi ned as a vector quantity), and β = ( e /2 m ) × ( h /2 π ) and is called the Bohr
magneton.